Many fixed and rotary-wing aircraft have parts, including but not limited to access covers, cowling panels, spinner cones, and fairings, that must be removably attached to other parts of the airframe. Typically, it is desirable to use screws for this removable attachment of such removable parts.
Many of these removable parts are attached in places where access is possible from only one side—there is a “blind” side inaccessible to wrenches, hands, and tools once the removable part is in place; the blind side often being accessible with the removable part removed or at some point during airframe assembly. Further, many of these removable parts are attached to portions of airframe made of sheet metal or plastic that is too thin to provide adequate engagement for screws.
Nutplates are frequently used to provide a female-threaded nut of adequate thickness to provide good engagement for a screw, to hold that nut in place on the blind side of airframe structure, and to prevent rotation of that nut during screw insertion and removal.
Nutplates are generally attached to airframe parts by riveting, but this is an expensive and time consuming process since countersinking of the rivets is often required to provide a smooth base for the removable part. Further, riveting of nutplates requires that additional holes be drilled into the airframe for those rivets, and each such hole slightly weakens the airframe. Nutplates have also been formed as spring-clips that require additional, large, or non-circular holes in the airframe.
U.S. Pat. No. 5,013,391 describes a nutplate having a base adapted for gluing to the blind side of a substrate. Screws for attaching the removable parts are threaded into this nutplate when the removable part is attached. Adhering a nutplate to a substrate is therefore known. In addition, some prior art systems have used Click Bond CB200-40 to adhere a nutplate to an airframe. Nevertheless, adhering a nutplate to an airframe has previously been unacceptable, or at least not preferable, due to the stresses and assembly requirements involved. For example, prior art systems using Click Bond CB200-40 have been measured to have a low lap shear strength at reduced temperatures (e.g., service temperatures of around −67 F), failure for those systems has been detected at approximately 50 pounds of push out force at −67 F, and those systems have been determined to have a relatively low safety factor (i.e., around 1.3) for hot and humid conditions (i.e., around 160 F with 95% relative humidity). As a result, prior art aircraft generally use rivets or spring-clips to attach nutplates.